Positioning system for use in lithographic apparatus

ABSTRACT

An H-drive arrangement for the substrate or mask stages of a lithographic apparatus has an X-beam  11  rigidly mounted to Y-sliders  121   a,    121   b  against X and Y translation and rotation about a Z axis (yaw) so as to form a rigid body in the XY plane. Rotation about X and Y axes is permitted in the joint between at least one Y-slider  121   a,    121   b  and X-beam  11.  Crash protection may be provided by a yaw rate sensor and/or resilient buffers which contact Y-beams  12   a,    12   b  in the event of out-of-range yaw motions.

[0001] The present invention relates to a positioning system, such asmay be used to position a moveable object table in three degrees offreedom. More particularly, the invention relates to the use of thepositioning system in a lithographic projection apparatus comprising:

[0002] an illumination system for supplying a projection beam ofradiation;

[0003] a first object table for holding a mask;

[0004] a second, movable object table for holding a substrate; and

[0005] a projection system for imaging an irradiated portion of the maskonto a target portion of the substrate.

[0006] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, catadioptric systems,and charged particle optics, for example. The illumination system mayalso include elements operating according to any of these principles fordirecting, shaping or controlling the projection beam of radiation. Inaddition, the first and second object tables may be referred to as the“mask table” and the “substrate table”, respectively.

[0007] Lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, the mask(reticle) may contain a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(comprising one or more dies) on a substrate (silicon wafer) which hasbeen coated with a layer of radiation-sensitive material (resist). Ingeneral, a single substrate will contain a whole network of targetportions which are successively irradiated via the mask, one at a time.In one type of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionin one go; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus—which is commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned from International PatentApplication WO 97/33205.

[0008] In general, apparatus of this type contained a single firstobject (mask) table and a single second object (substrate) table.However, machines are becoming available in which there are at least twoindependently movable substrate tables; see, for example, themulti-stage apparatus described in International Patent Applications WO98/28665 and WO 98/40791. The basic operating principle behind suchmulti-stage apparatus is that, while a first substrate table isunderneath the projection system so as to allow exposure of a firstsubstrate located on that table, a second substrate table can run to aloading position, discharge an exposed substrate, pick up a newsubstrate, perform some initial metrology steps on the new substrate,and then stand by to transfer this new substrate to the exposureposition underneath the projection system as soon as exposure of thefirst substrate is completed, whence the cycle repeats itself; in thismanner, it is possible to achieve a substantially increased machinethroughout, which in turn improves the cost of ownership of the machine.

[0009] In a known lithographic apparatus, the drive unit of thepositioning mechanism for the substrate table comprises two linearY-motors each of which comprises a stator which extends parallel to theY-direction and is secured to a base of the positioning mechanism, and atranslator (Y-slide) which can be moved along the stator. The base issecured to the frame of the lithographic device. The drive unit furthercomprises a linear X-motor which includes a stator which extendsparallel to the X-direction, and a translator (X-slide) which can bemoved along the stator. The stator is mounted on an X-beam which issecured, near its respective ends, to the translators of the linearY-motors. The arrangement is therefore H-shaped, with the two Y-motorsforming the “uprights” and the X-motor forming the “cross-piece”, andthis arrangement is often referred to as an H-drive or gantry. U.S. Pat.No. 4,655,594 describes such an arrangement using hydraulic linearmotors and mentions the possibility of using electric linear motors.

[0010] The driven object, in this case the substrate table, is providedwith a so-called air foot. The air foot comprises a gas bearing by meansof which the substrate table is supported so as to be movable over aguide surface of the base extending at right angles to the Z-direction.

[0011] To enable such an H-drive to actively control the yaw (rotationabout the Z-axis) of the driven object, the two linear Y-motors aredriven independently and the X-beam is usually mounted to theY-translators by pivots (though U.S. Pat. No. 4,655,594 suggests that arigid joint can be used). However, in this arrangement, very high loadsare experienced at the pivots between the X-beam and the Y-slides. Thepivots have to carry not only thrust reactions from the X-motor throughthe side bearings to the surrounding structure, but also the Y-motoractuation forces. This places very high demands on the elastic hingescommonly used for such pivots, especially when the yaw motion range isrelatively large.

[0012] Further problems are encountered as the pivots on the X-beamcannot always be positioned on the line of force for the Y-motors, sothat the side thrust bearing of the Y-slide has to accommodate both theX-reaction forces as well as the moment created by the Y-actuator forcesand the offset between the pivots and the Y line of force. The resultinghigh loads in the known arrangement therefore leads to a design whichcan be cumbersome and heavy.

[0013] An object of the present invention is to provide an improvedpositioning apparatus which avoids or alleviates the problems of knownpositioning apparatus.

[0014] According to the present invention there is provided alithographic projection apparatus for imaging of a mask pattern in amask onto a substrate provided with a radiation sensitive layer, theapparatus comprising:

[0015] an illumination system for supplying a projection beam ofradiation;

[0016] a first object table for holding a mask;

[0017] a second object table for holding a substrate;

[0018] a projection system for imaging irradiated portions of the maskonto target portions of the substrate;

[0019] a positioning system for positioning at least one of said objecttables in a plane, said positioning system comprising:

[0020] first and second generally parallel side-beams having respectivefirst and second sliders mounted thereon;

[0021] first and second motor means for moving said first and secondsliders longitudinally of their respective side beams;

[0022] a cross-beam mounted near first and second ends thereof to saidfirst and second sliders respectively and having a third slider mountedthereon, said cross-beam and said first and second sliders being mountedtogether so as to form a body that is substantially rigid in translationin said plane and in rotation about an axis normal to said plane;

[0023] third motor means for moving said third slider longitudinally ofsaid cross-beam, said third slider having an object holder for holdingsaid one object table; characterized by:

[0024] a thrust bearing pivotally mounted to said first slider fortransmitting forces in said plane and perpendicular to said first sidebeam between said cross-beam and said first side beam.

[0025] By mounting the cross-beam (X-beam) and first and second (Y-)sliders rigidly against rotation about an axis (the Z-axis) normal tothe plane of movement of the moveable object (XY-plane) as well asagainst translations in that plane, the X-beam and Y-sliders form arigid body in the XY-plane. This eliminates the need for pivots capableof transmitting the Y actuation forces and X reaction forces to and fromthe X-beam and simplifies the construction of the apparatus.

[0026] Further, the thrust bearing pivotally mounted between the sliderand side beam thereby transfers forces in the (nominal) X-direction tothe side beam without experiencing any forces in the Y-direction,simplifying the construction of the bearing and pivot. There is also no“cross talk” between X and Y forces; the transfer of the X-directionforces does not give rise to any Y-direction forces.

[0027] The motors driving the first and second (Y-) sliders may belinear motors having a stator mounted on the beam and an armature in theslider. The motors may be arranged to provide substantially constantcharacteristics independent of the angular (yaw) position of theY-sliders, for example by having an armature of magnets arranged in aherring-bone pattern, or the driving software or hardware may bearranged to compensate for yaw-dependent properties of the motor.

[0028] Damage protection can advantageously be provided by a yaw and/ora yaw rate sensor and cutoff arranged to cut power to the motors in theevent of an excessive yaw or rate of yaw of the X-beam. Resilientbuffers arranged to contact the Y-beams in the event of out-of-range yawmotions can provide additional protection.

[0029] According to a yet further aspect of the invention there isprovided a method of manufacturing a device using a lithographicprojection apparatus comprising:

[0030] a radiation system for supplying a projection beam of radiation;

[0031] a first movable object table provided with a mask holder forholding a mask;

[0032] a second movable object table provided with a substrate holderfor holding a substrate; and

[0033] a projection system for imaging irradiated portions of the maskonto target portions of the substrate; the method comprising the stepsof:

[0034] providing a mask bearing a pattern to said first moveable objecttable;

[0035] providing a substrate provided with a radiation-sensitive layerto said second movable object table;

[0036] irradiating portions of the mask and imaging said irradiatedportions of the mask onto said target portions of said substrate;characterized in that:

[0037] a positioning apparatus used to position one of said movableobject tables prior to or during said steps of irradiating and imagingcomprises:

[0038] first and second generally parallel side-beams having respectivefirst and second sliders mounted thereon;

[0039] first and second motor means for moving said first and secondsliders longitudinally of their respective side beams;

[0040] a cross-beam mounted near first and second ends thereof to saidfirst and second sliders respectively and having a third slider mountedthereon, said cross-beam and said first and second sliders being mountedtogether so as to form a body that is substantially rigid in translationin said plane and in rotation about an axis normal to said plane;

[0041] third motor means for moving said third slider longitudinally ofsaid cross-beam, said third slider having an object holder for holdingsaid moveable object; characterized by:

[0042] a thrust bearing pivotally mounted to said first slider fortransmitting forces in said plane and perpendicular to said first sidebeam between said cross-beam and said first side beam.

[0043] In a manufacturing process using a lithographic projectionapparatus according to the invention a pattern in a mask is imaged ontoa substrate which is at least partially covered by a layer ofradiation-sensitive material (resist). Prior to this imaging step, thesubstrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake (PEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. an IC. Such a patterned layer may then undergo variousprocesses such as etching, ion-implantation (doping), metallisation,oxidation, chemo-mechanical polishing, etc., all intended to finish offan individual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

[0044] Although specific reference may be made in this text to the useof the apparatus according to the invention in the manufacture of ICs,it should be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion” or “exposure area”, respectively.

[0045] In the present document, the terms radiation and projection beamare used to encompass all types of electromagnetic radiation or particleflux, including, but not limited to, ultraviolet radiation (e.g. with awavelength of 365, 248, 193, 157 or 126 nm), EUV, X-rays, electrons andions.

[0046] The present invention will be described below with reference toexemplary embodiments and the accompanying schematic drawings, in which:

[0047]FIG. 1 depicts a lithographic projection apparatus according to afirst embodiment of the invention;

[0048]FIG. 2 is a plan view of the wafer stage, comprising the substratetable and drive unit, of the apparatus of FIG. 1;

[0049]FIG. 3 is an enlarged side view of the X-beam of the wafer stageof FIG. 2;

[0050]FIG. 4 is an enlarged plan view of the X-beam of the wafer stageof FIG. 2;

[0051]FIG. 5 is a plan view of a part of a wafer stage according to asecond embodiment of the invention;

[0052]FIG. 6 is a side view of a part of the wafer stage of FIG. 5;

[0053]FIG. 7 is an enlarged, partly sectioned, view of crash pins in thewafer stage of FIG. 5;

[0054]FIG. 8 is an enlarged side view of a part of the wafer stage ofFIG. 2;

[0055]FIG. 9 is a plan view of the part of the wafer stage of FIG. 8;

[0056]FIG. 10 is a plan view of a bearing arrangement used in a thirdembodiment of the invention;

[0057]FIG. 11 is a side view of the bearing arrangement of FIG. 10;

[0058]FIG. 12 is a side view of one end of the X-beam of a fourthembodiment of the invention showing a collision prevention mechanism;and

[0059]FIGS. 13 and 14 are underneath pan views of the end of the X-beamof the fourth embodiment in a normal and an excessive yaw positionrespectively.

[0060] In the drawings, like reference numerals indicate like parts.

[0061] Embodiment 1

[0062]FIG. 1 schematically depicts a lithographic projection apparatusaccording to the invention. The apparatus comprises:

[0063] a radiation system LA, IL for supplying a projection beam PB ofradiation (e.g. UV or EUV radiation);

[0064] a first object table (mask table) MT provided with a mask holderfor holding a mask MA (e.g. a reticle), and connected to firstpositioning means for accurately positioning the mask with respect toitem PL;

[0065] a second object table (substrate table) WT provided with asubstrate holder for holding a substrate W (e.g. a resist-coated siliconwafer), and connected to second positioning means for accuratelypositioning the substrate with respect to item PL;

[0066] a projection system (“lens”) PL (e.g. a refractive orcatadioptric system, a mirror group or an array of field deflectors) forimaging an irradiated portion of the mask MA onto a target portion C ofthe substrate W.

[0067] As here depicted, the apparatus is of a transmissive type (i.e.has a transmissive mask). However, in general, it may also be of areflective type, for example.

[0068] In the example depicted here, the radiation system comprises asource LA (e.g. a Hg lamp, excimer laser, a laser or discharge plasmasource, an undulator provided around the path of an electron beam in astorage ring or synchrotron, or an electron or ion beam source) whichproduces a beam of radiation. This beam is passed along various opticalcomponents comprised in the illumination system IL,—e.g. beam shapingoptics Ex, an integrator IN and a condenser CO—so that the resultantbeam PB has a desired shape and intensity distribution.

[0069] The beam PB subsequently intercepts the mask MA which is held ina mask holder on a mask table MT. Having passed through the mask MA, thebeam PB passes through the lens PL, which focuses the beam PB onto atarget portion C of the substrate W. With the aid of the interferometricdisplacement measuring means IF, the substrate table WT can be movedaccurately by the second positioning means, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning means can be used with the aid of interferometricdisplacement measuring means to accurately position the mask MA withrespect to the path of the beam PB, e.g. after mechanical retrieval ofthe mask MA from a mask library. In general, movement of the objecttables MT, WT will be realized with the aid of a long stroke module(course positioning) and a short stroke module (fine positioning), whichare not explicitly depicted in FIG. 1.

[0070] The depicted apparatus can be used in two different modes:

[0071] 1. In step mode, the mask table MT is kept essentiallystationary, and an entire mask image is projected in one go (i.e. asingle “flash”) onto a target portion C. The substrate table WT is thenshifted in the x and/or y directions so that a different target portionC can be irradiated by the beam PB;

[0072] 2. In scan mode, essentially the same scenario applies, exceptthat a given target portion C is not exposed in a single “flash”.Instead, the mask table MT is movable in a given direction (theso-called “scan direction”, e.g. the x direction) with a speed v, sothat the projection beam PB is caused to scan over a mask image;concurrently, the substrate table WT is simultaneously moved in the sameor opposite direction at a speed V=Mv, in which M is the magnificationof the lens PL (typically, M=¼ or ⅕). In this manner, a relatively largetarget portion C can be exposed, without having to compromise onresolution.

[0073]FIG. 2 shows the wafer stage of the lithographic apparatus of thefirst embodiment in plan. The wafer W is mounted on substrate (wafer)table WT which is positioned by a coarse positioning mechanism (longstroke module) generally indicated as 10. The coarse positioningmechanism has a generally H-shaped configuration in which the cross baris formed by X-beam 11 and the uprights by Y-beams 12 a, 12 b. Thesebeams are so called because they are generally parallel to theorthogonal X and Y axes of a reference coordinate system defined for theapparatus.

[0074] It should be noted that the wafer table WT may incorporatefurther positioning systems cascaded to the course positioning mechanismto accurately control the position of the wafer in any or all of the sixpossible degrees of freedom. The working of such a fine positioningsystem is not particularly relevant to the present invention and adescription thereof is therefore omitted for the sake of brevity.

[0075] Wafer table WT is supported on X-beam 11 by X-slider 111 whichincludes a linear motor acting against magnet track 112 enabling wafertable WT to be displaced linearly along X-beam 11. In alternativeembodiments of the invention, wafer table WT may simply be driven in X,Y and Rz by X-slider 111 and supported separately, e.g. by an air-footover a guide surface of the machine frame or the X-beam 11, in Z, Rx andRy. X-beam 11 is mounted near its ends to respective Y-sliders 121 a,121 b, which similarly to X-slider 111, include linear motors actingagainst magnet tracks 122 a, 122 b enabling the beam to be displacedalong the Y-direction.

[0076] Displacement of X-slider 111 longitudinally of X-beam 11 anddisplacement of X-beam 11 in the Y-direction allows wafer table WT to bepositioned coarsely in the X-Y plane. Independent control of Y-sliders121 a, 121 b allows the rotational position of wafer table WT about theZ-axis (yaw) to be controlled within a certain range.

[0077] According to the present invention, the X-beam is coupled toY-sliders 121 a, 121 b rigidly in at least the X and Y directions andagainst rotation about the Z axis, Rz, so as to form a rigid body in theX-Y plane. This eliminates the need for Rz pivots between X-beam 11 andY-sliders 121 a, 121 b and ensures direct coupling of the Y-directionactuation forces to X-beam 11. As described further below, the couplingof the X-beam to the Y-sliders may also be rigid in Z and one side mayalso be rigid in Rx. Preferably neither side is coupled rigidly in Ry.

[0078] X-slider 111 is box shaped and surrounds X-beam 11. Alternativelyit may take the form of an inverted U placed over the top of the X-beam11. The X-slider 111 is supported by opposed-pad air (gas) bearings soas to be displaceable longitudinally of X-beam 11 (in the nominalX-direction) substantially without friction but is restrained relativeto the X-beam so that it cannot move in the Y- and Z-directions, noryaw. X-beam 11 itself is a multi-cell hollow beam with at least threelongitudinally extending cells, the middle one of which isasymmetrically offset in the Z-direction to accommodate the stationarypart, e.g. magnet track 112 or a coil assembly, of the X-motor so thatthe driving force of the X-motor is as close as possible to the centerof gravity of the moving mass. X-slider 111 and X-beam 11 can be made ofa technical ceramic material, such as Al₂O₃, SiC, SiSiC, CSiC, etc., toensure they have relatively high Eigen-frequencies.

[0079] Reaction forces in the nominal X-direction, i.e. the X componentof reaction forces generated by displacement of X-slider 111 and wafertable WT along X-beam 11, are transferred to Y-beam 12 a. This iseffected by side thrust bearing 123 a which is connected to Y-slider 121a via pivot 124 a and acts against upstanding wall 125 a provided on theoutside edge of Y-beam 12 a. The side thrust bearing 123 a may comprisea single-sided aerostatic thrust bearing with a magnetic or vacuumpre-load of magnitude sufficiently larger, including safety factors,than the maximum reaction force expected in use of the positioningmechanism. An alternative would be a double-sided (opposed pad) airbearing acting on opposite faces of wall 125 a.

[0080] A second side bearing 123 b is provided on Y-slider 121 b tomount an encoder reading head 127 b, which is discussed further below.Side bearing 123 b is mounted to Y-slider 121 b via pivot 124 b whichincludes a leaf spring arrangement or linear bearing, such as across-roller guideway, to give freedom of movement in the X-direction.This accommodates the reduction in effective length of X-beam 11 in theX-direction as yaw angle increases (so-called cosine foreshortening) andensures that side bearing 123 b remains in contact with wall 125 b.

[0081] As shown in FIG. 8, which is a side view of one end of X-beam 11,Y-slider 121 and side bearing 123, and FIG. 9, which is a plan view ofthose components, side bearing 123 comprises a yoke member 30 which ismounted to Y-slider 121 via pivot 124 and carries bearings 31. Yokemember 30 comprises spars 30 a, 30 b which extend horizontally from theupper corners of plate 30 c to pivot 124. Plate 30 c extends verticallydownwards between Y-slider 121 and wall 125 attached to Y-beam 12 andcarries bearings 31 which act against wall 125. The center line ofbearings 31 is thereby arranged to be in line with the X-reaction forcesgenerated by translations of the X-slider (not shown) along X-beam 11.

[0082] Because X-beam 11 and Y sliders 121 a, 121 b form a rigid body inthe XY plane, if X-beam 11 is displaced from parallel to the X axis toeffect yaw positioning of wafer table WT, the linear motors of Y-sliders121 a and 121 b will be correspondingly rotated relative to their magnettracks 122 a, 122 b. If the Y-motors are of a conventional type withiron armatures and a simple slanted arrangement of magnets in the track,the resultant changes in motor constant and cogging force can becompensated for in software. Alternatively, multi-phase Lorentz-typeironless linear motors can be used which do not result in significantchanges in motor performance with yaw angle. A further alternative is touse magnet tracks in which the magnets form a herring-bone pattern. Insuch an arrangement, the changes in motor constant and cogging force onone side of the herring-bone almost exactly cancel those on the oppositeside, resulting in a motor assembly substantially insensitive to yaw.

[0083] As shown in FIG. 4, X-beam 11 is connected to Y-sliders 121 a,121 b by joints 126 a, 126 b. Joints 126 a, 126 b are arranged toprovide freedom for roll (rotation Ry) between X-beam 11 and Y-sliders121 a, 121 b in order to accommodate any deviations from parallel in theupper bearing surfaces of Y-beams 12 a, 12 b. Such deviations may occurbecause of a height difference or misalignment between the two beams.The angular range of the required freedom is limited and can be providedby elastic flexures (such as so-called cross-pivots) or by normal rotarybearings (such as roller or ball bearings).

[0084] One of joints 126 a, 126 b, in this embodiment joint 126 a, isarranged to be rigid against rotation (Rx) about an axis parallel to theX-direction so as to support X-beam 11 against pitch. The other, in thisembodiment joint 126 b, is arranged to provide considerable freedom forpitch movements between X-beam 11 and Y-slider 121 b. This avoids anytorsion forces in X-beam 11 which might otherwise be induced bydeviations from parallel in the two Y-beams 12 a, 12 b. The Rx freedomin joint 126 b is provided by a simple pivot, elastic or other means,between X-beam 11 and Y-slider 121 b so that the two Y-sliders 121 a,121 b are substantially identical and to ensure that encoder head 127 bremains parallel to its linear grating 128 b. An alternative to thepivot would be a vertical bearing arrangement which supports theY-slider 121 b on the side beam 12 b which has load capacity in thevertical direction but negligible stiffness against pitch and roll.

[0085] To determine the positions of Y-sliders 121 a, 121 b, incrementalencoders 127 a, 127 b and linear gratings 128 a, 128 b, mounted onY-beams 12 a, 12 b, are provided. Incremental encoders 127 a, 127 b mayconveniently be mounted on side bearings 123 a, 123 b and so willmaintain their orientation relative to gratings 128 a, 128 b.Alternatively, they may be mounted on Y-sliders 121 a, 121 b and thecosine foreshortening caused by yaw movements of X-beam 11 compensatedfor by the provision of a mechanism such as a linear bearing or leafspring arrangement.

[0086] For motor commutation purposes, it is necessary to know theY-positions of Y-sliders 121 a, 121 b along the center lines of themotors. With encoders 127 a, 127 b mounted on side bearings 123 a, 123b, the center line positions can be obtained directly by positioningpivots 124 a, 124 b exactly on the center lines. Alternatively, thecenter line positions can be obtained from a hardware or softwareinterpolation algorithm, knowing the distances between the motor centerpoints and the bearing pivot points. This alternative providesconsiderable additional flexibility in the mechanical layout of the sidebearing arrangements.

[0087] Crash protection in the X and Y direction is provided by simpleelastic (e.g. pre-loaded helical or conical springs) or viscous (e.g.hydraulic dampers) devices or a combination of the two.

[0088] Crash protection in yaw requires high moment loads on the wholepositioning mechanism to be restrained by a system of forces actingbetween Y-sliders 121 a, 121 b and Y-beams 12 a, 12 b as the latterprovide the only connection to the real world. To prevent excessive yawcorrection forces in the X-direction causing side bearings 123 a, 123 bto be pulled away from their bearing surfaces, a yaw rate sensor 113 isprovided on X-beam 11, X-slider 111 or wafer table WT. If a rate of yawexceeding a preset safety limit is detected, a hardwired protectioncircuit is triggered to switch off all motors to prevent furtherincrease in rotational kinetic energy. The rotational kinetic energypresent prior to motor shutdown can be absorbed in a controlled crashvia elastic and/or viscous dampers 114 mounted on X-beam 11 so as toengage the sides of Y-beams 12 a, 12 b.

[0089] Alternatively, the incremental encoders 127 a, 127 b and lineargratings 128 a, 128 b can be used to determine the yaw and the rate ofyaw. If the yaw, the rate of yaw or a combination of both is detectedwhich exceeds a pre-set safety limit all motors can be switched off toprevent a further increase of rotational energy.

[0090] Embodiment 2

[0091] FIGS. 5 to 7 show a part of a second embodiment of the inventionwhich has additional arrangements for crash prevention. Only one side ofthe apparatus is shown; the other is similar. Parts not shown or notspecifically described below may be similar to corresponding parts ofthe first embodiment.

[0092] As shown in FIGS. 5 and 6, a crash bar 20 is mounted belowY-slider 121 adjacent the connection to X-beam 11. Crash bar 20 extendsin the Y-direction either side of X-beam 11 and carries at each end twocrash pins 21. Crash pins 21 protrude from crash bar 20 towards Y-beam12 and are positioned side by side in the Y-direction. In otherembodiments they may be positioned one above the other or diagonally.

[0093]FIG. 7 is a partly cross-sectional enlarged view of two crash pins21. It will be seen that each crash pin 21 comprises a generallycylindrical head portion 21 a, a flange 21 b provided around theproximal end of head portion 21 a and a rod portion 21 c extendingcoaxially with the head portion 21 a away therefrom. A cylindrical bore22 is provided through crash bar 20 for each crash pin 21. Each bore 22extends generally in the X-direction and has a portion 22 a ofrelatively small diameter adjacent Y-beam 12 joined by shoulder 22 b toa portion 22 c of relatively large diameter on the side away from Y-beam12. Crash pin 21 is inserted into bore 22 from the side away from Y-beam12 so that head portion 21 a projects towards Y-beam 12 through portion22 a but is prevented from passing completely through bore 22 byengagement of flange 21 b with shoulder 22 b.

[0094] A resilient member 23, e.g. a helical spring, is provided aroundrod portion 21 c and the end of bore 22 is closed by plug 24 which has acentral through-hole 24 a through which rod portion 21 c projects.Resilient member 23 acts against plug 24 normally to urge pin 21 towardY-beam 12. The uncompressed length of resilient member 23 and thedimensions of bore 21 are chosen to provide a desired pre-loading to thecrash pin.

[0095] The dimensions of crash bar 20, the position of crash pins 21 andthe protruding length of head portion 21 a are chosen so that if the yawof X-beam 11 exceeds a safe or permissible amount, the crash pins willcome into contact with the side of Y-beam 12 before any other part ofthe yawing assembly, i.e X-beam 11, Y-sliders 121 and the othercomponents mounted thereon, meets an obstacle. Crash pins 21 will bedepressed by continued yawing of X-beam 11 against the resilience ofresilient member 23 so that the crash pins act as buffers to provide a“soft-landing” for the yawing assembly.

[0096] Resilient member 23 may be substantially elastic or may include asignificant amount of plasticity or friction to reduce rebound. Viscousor other forms of damper may also be included. The relative positionsand lengths of the pins on each end of crash bar 20, the moduli ofresilient members 23 and the degree of pre-loading provided may bevaried so that the pins come into contact with Y-beam 12 simultaneouslyor sequentially and so as to provide uniform or progressive resistanceto yaw once contact has been made.

[0097] Embodiment 3

[0098]FIGS. 10 and 11 show a third embodiment of the invention whichdiffers from the first and second embodiments in the arrangement of theside thrust bearing. Only one side of embodiment 3 is shown, the othermay be similar or may lack a side bearing or may include anX-translation mechanism to accommodate cosine fore-shortening, asdiscussed above. Parts not shown in FIGS. 10 and 11 or not specificallydiscussed below may be similar to corresponding parts in the first andsecond embodiments.

[0099] In embodiment 3, the side bearings 123 are connected to theY-sliders 121 by a leaf-spring arrangement 150. The leaf springscomprised in arrangement 150 are generally vertical so as to besubstantially rigid in Z and angled so that they define an effective,virtual pivot point 124′. Virtual pivot point 124′ is preferablyarranged so as be over the center line of the Y-motor track 122.

[0100] Embodiment 4

[0101] The fourth embodiment, which may be the same as any of the firstto third embodiments save as described below, has a crash preventionmechanism 200, shown in FIGS. 12 to 14, which uses a torsion rod toabsorb energy in the event of an excessive yaw motion.

[0102] As shown in FIG. 12, the X-beam 11 of the fourth embodiment isconnected to Y-slider 121 via coupling member 201 which projects belowthe Y-beam 12. A framework 202 extends horizontally from coupling member201 underneath Y-beam 12 to support a torsion rod 204 which is elongatein the Y-direction. Torsion rod 204 has a bearing 205 at each end; thesebearings 205 are rigidly connected to the torsion rod and project into agroove 206 provided in the under surface of Y-beam 12.

[0103] When the yaw (R_(z) position) of the X-beam 11 is withinacceptable limits there is a clearance between the bearings 205 and theside walls of groove 206; this can be seen in FIG. 13. However, when theyaw of the X-beam 11 becomes excessive, the bearings 205 will come intocontact with the side walls of groove 206 as shown in FIG. 14. Continuedyawing R_(z) of the X-beam 11 will cause reaction forces F₁, F₂ to beextended on the bearings 205. Reaction forces F₁, F₂ are in oppositedirections and so a torque is extended on torsion rod 204. Torsion rod204 is allowed to twist, at least to a limited degree, relative toframework 202, 203 and in doing so absorbs energy and counteracts theR_(z) motion of the X-beam 11.

[0104] FIGS. 12 to 14 show a collision prevention mechanism 200 providedat one end of X-beam 11. Depending on the masses and yaw rates expectedin use, a second similar collision prevention mechanism may also beprovided at the other end.

[0105] In the various embodiments of the invention, it is preferablethat the centers of gravity of the moving bodies, the lines of action ofthe various drive forces and the pivot points in the various couplings,all lie close, for example within ±20 mm, to a single XY plane.

[0106] Whilst we have described above a specific embodiment of theinvention it will be appreciated that the invention may be practicedotherwise than described. The description is not intended to limit theinvention. In particular it will be appreciated that the invention maybe used to position either or both mask and substrate tables of alithographic apparatus.

1. A lithographic projection apparatus for imaging of a mask pattern ina mask onto a substrate provided with a radiation sensitive layer, theapparatus comprising: an illumination system for supplying a projectionbeam of radiation; a first object table for holding a mask; a secondobject table for holding a substrate; a projection system for imagingirradiated portions of the mask onto target portions of the substrate; apositioning system for positioning at least one of said object tables ina plane, said positioning system comprising: first and second generallyparallel side-beams having respective first and second sliders mountedthereon; first and second motor means for moving said first and secondsliders longitudinally of their respective side beams; a cross-beammounted near first and second ends thereof to said first and secondsliders respectively and having a third slider mounted thereon, saidcross-beam and said first and second sliders being mounted together soas to form a body that is substantially rigid in translation in saidplane and in rotation about an axis normal to said plane; third motormeans for moving said third slider longitudinally of said cross-beam,said third slider having an object holder for holding said one objecttable; characterized by: a thrust bearing pivotally mounted to saidfirst slider for transmitting forces in said plane and perpendicular tosaid first side beam between said cross-beam and said first side beam.2. Apparatus according to claim 1 further comprising a second thrustbearing pivotally mounted to said second slider for transmitting forcesin said plane and perpendicular to said second side beam between saidcross-beam and said second side beam, said second thrust bearing havingmeans to accommodate the effective reduction in length of saidcross-beam perpendicular to said side beams with rotation of saidcross-beam about an axis normal to said plane.
 3. Apparatus according toclaim 1 or 2 wherein said cross-beam is mounted to at least one of saidfirst and second sliders by a joint allowing at least some relativerotation about at least one axis parallel to said plane.
 4. Apparatusaccording to claim 1 , 2 or 3 wherein said first and second motor meanseach comprises a linear motor having a stator mounted on the respectiveside-beam and an armature mounted on the respective slider.
 5. Apparatusaccording to claim 4 wherein each said linear motor is adapted toprovide driving forces that are substantially independent of angularposition of said sliders at least within a permitted range of movementof said cross-beam.
 6. Apparatus according to claim 5 wherein each saidstator comprises a magnet track having magnets arranged in aherring-bone pattern.
 7. Apparatus according to claim 4 furthercomprising control means for controlling said linear motors to drivesaid first and second sliders to desired positions, said control meansbeing adapted to vary the drive signal applied to said linear motors tocompensate for the angular position of said first and second slidersrelative to said side-beams to provide desired drive forces. 8.Apparatus according to any one of the preceding claims furthercomprising a rotation sensor mounted on said cross beam, said thirdslider or said object holder for detecting rotation about an axis normalto said plane and cut-off means responsive to said rotation sensor forcutting power to said first and second motor means in the event that therate of rotation detected by said rotation detector exceeds apredetermined value.
 9. Apparatus according to claim 8 wherein saidcutoff means is hardwired to the power supply to said first and secondmotor means.
 10. Apparatus according to any one of the preceding claimsfurther comprising resilient crash protection means mounted to at leastone of said first and second sliders and said cross-beam so as come intocontact with at least one of said side beams in a crash event wherebythe angular position of said cross-beam goes outside a permitted rangeand to restrain further rotation of said cross beam.
 11. Apparatusaccording to claim 10 wherein said resilient crash protection meanscomprises an elongate member generally perpendicular to said cross-beamand having at least one projecting pin resiliently mounted near one endthereof, said pin projecting towards one of said side-beams so as tocontact it in said crash event and to resist further rotation of saidcross-beam.
 12. Apparatus according to claim 11 wherein said projectingpin is mounted so as to be depressed into said elongate member againstthe biasing force of a resilient member on said further rotation. 13.Apparatus according to claim 10 wherein said resilient crash protectionmeans comprises an elongate torsion rod having spaced apart bearingsarranged to contact opposite side walls of a groove provided in one ofsaid side beams in a said crash event.
 14. A method of manufacturing adevice using a lithographic projection apparatus comprising: a radiationsystem for supplying a projection beam of radiation; a first movableobject table provided with a mask holder for holding a mask; a secondmovable object table provided with a substrate holder for holding asubstrate; and a projection system for imaging irradiated portions ofthe mask onto target portions of the substrate; the method comprisingthe steps of: providing a mask bearing a pattern to said first moveableobject table; providing a substrate provided with a radiation-sensitivelayer to said second movable object table; irradiating portions of themask and imaging said irradiated portions of the mask onto said targetportions of said substrate; characterized in that: a positioningapparatus used to position one of said movable object tables prior to orduring said steps of irradiating and imaging comprises: first and secondgenerally parallel side-beams having respective first and second slidersmounted thereon; first and second motor means for moving said first andsecond sliders longitudinally of their respective side beams; across-beam mounted near first and second ends thereof to said first andsecond sliders respectively and having a third slider mounted thereon,said cross-beam and said first and second sliders being mounted togetherso as to form a body that is substantially rigid in translation in saidplane and in rotation about an axis normal to said plane; third motormeans for moving said third slider longitudinally of said cross-beam,said third slider having an object holder for holding said moveableobject; characterized by: a thrust bearing pivotally mounted to saidfirst slider for transmitting forces in said plane and perpendicular tosaid first side beam between said cross-beam and said first side beam.15. A device manufactured according to the method of claim 14 .
 16. Apositioning apparatus for positioning a moveable object translationallyand rotationally in a plane, the apparatus comprising: first and secondgenerally parallel side-beams having respective first and second slidersmounted thereon; first and second motor means for moving said first andsecond sliders longitudinally of their respective side beams; across-beam mounted near first and second ends thereof to said first andsecond sliders respectively and having a third slider mounted thereon,said cross-beam and said first and second sliders being mounted togetherso as to form a body that is substantially rigid in translation in saidplane and in rotation about an axis normal to said plane.; third motormeans for moving said third slider longitudinally of said cross-beam,said third slider having an object holder for holding said moveableobject; characterized by: a thrust bearing pivotally mounted to saidfirst slider for transmitting forces in said plane and perpendicular tosaid first side beam between said cross-beam and said first side beam.